Record-Breaking Laser Could Help Prove Einstein’s Work

When Albert Einstein published his general theory of relativity in 1915, he probably didn’t expect that for the next 100 years, scientists around the world would continually find different ways to prove his remarkable equations, which define a unified approach to describe and link gravity to the geometric properties of space and time.  

One experiment in particular that he may not have considered would have been the transmission of a laser signal through Earth’s atmosphere into space, in order to create the world’s most precise method for comparing the flow of time between two separate locations.

Yet, that is exactly what scientists from the International Centre for Radio Astronomy Research (ICRAR) and the University of Western Australia (UWA) are hoping to achieve, after succeeding in developing technology that has produced the world’s most stable transmission through our planet’s atmosphere. Their results have been published in the journal Nature Communications today.

Working as part of an international collaboration with researchers from the French National Centre for Space Studies (CNES) and the French metrology lab Systèmes de Référence Temps-Espace (SYRTE) at Paris Observatory, the team was able to use ‘phase stabilisation’ technology (developed here in Australia by ICRAR and UWA scientists) and couple this with advanced self-guiding terminals to send a laser signal from one point to another without interference from the atmosphere.

Effectively, removing any form of atmospheric turbulence that could interfere with the signal.

“We can correct for atmospheric turbulence in 3D, that is, left-right, up-down and, critically, along the line of flight,” said lead author Mr. Benjamin Dix-Matthews, a Ph.D. student at ICRAR and UWA. “It’s as if the moving atmosphere has been removed and doesn’t exist.”

A Laser (the term an acronym for Light Amplification of Stimulated Electromagnetic Radiation) is a source of light generated by a device with special properties such as spatial coherence, allowing it to be focused to a small region, and remain as a ‘beam’ over great distances.

First built in the 1960s, lasers have become an integral part of our societies, with a wide range of purposes across manufacturing, medical, metrology, data storage, communications, scientific purposes, energy, navigation, and military applications.

When sending a laser beam through the atmosphere (in this case, for scientific testing purposes), the movement of the atmosphere causes the transmitted signal to travel slightly different distances with each time interval. So, when the team from ICRAR and UWA were measuring the returning signal, their instruments were detecting slightly different time-delays, known as ‘phase delays’.

To get around this, a technique known as phase stabilisation is introduced, in which rapid measurements of the turbulence are taken, and accounted for as part of the transmitted signal.

“For our phase stabilisation we measure the slight phase change caused by the atmosphere, and then actively change the phase of the transmitted signal by the opposite amount. Our phase shift and the atmospheric phase shift will then cancel each other out when the signal is received,” said Ben.

This process has helped stabilise the effects of the dynamic atmosphere, which in turn leads to some interesting science that can be achieved with the technology.

“It allows us to send highly-stable laser signals through the atmosphere while retaining the quality of the original signal.”

Lasers play an integral part in the space sciences today in a number of different fields, such as astronomical spectroscopy, remote sensing onboard spacecraft visiting planets, adaptive optics in large telescopes, communications, and interferometry (like the lasers used by LIGO to detect gravitational waves), through to space-based services closer to Earth, such as military and meteorology applications.

But this new technology can start to delve into answers about some of the underlying physics of our Universe, that is yet to be explored – like features of Einstein’s general relativity that could be exploited, to provide real-world applications for the benefit of many.

ICRAR-UWA senior researcher Dr. Sascha Schediwy said the research has exciting applications.

“For instance, this technology could improve satellite-based studies of how the water table changes over time, or to look for ore deposits underground,” he said. “If you have one of these optical terminals on the ground and another on a satellite in space, then you can [also] start to explore fundamental physics.”

“Everything from testing Einstein’s theory of general relativity more precisely than ever before, to discovering if fundamental physical constants change over time.”

“You could use this technology to compare two optical atomic clocks through the atmosphere. Atomic clocks are so stable now that you can see the timing difference caused by the change in the gravitational field caused by moving a clock up or down by 1cm. This makes possible the use of atomic clocks as new sensors that can be used to directly measure differences in gravitational field strength,” added Ben.

Whilst the equations of general relativity might seem like an abstract concept to most people, the core principles are inherently (and subtly) attached to our everyday lives – through the usage of services like GPS in our mobile devices, financial systems, and agricultural practices.

By improving the accuracy of gravitational sensors (such as the usage of extremely accurate clocks that utilise this technology), then GPS services would also improve drastically. This may not seem immediately essential to the ongoing practices we participate in on a daily basis (after all, current GPS works fairly well), but when we think about the accuracy required for self-driving vehicles or sensitive remote operations, the benefits start to exponentially increase.

Communication experts around the world have recognised an oncoming risk – that transmissions made using the radio portion of the spectrum are limited in terms of bandwidth and remaining available frequencies. This is said to only become more crowded as enormous satellite constellations like Starlink (SpaceX), with its tens of thousands of satellites, migrate into orbit.

So much so, that the former Australian Space Agency head commented in 2019 how Australia could become a world leader in migrating from radio communication methods to optical infrastructure, with communication technology and grounds stations already being prototyped.

Some of the benefits of laser communications, in particular for space-based applications, is the availability to send vastly larger amounts of data (securely) through this method (relative to radio transmissions).

“Our technology could help us increase the data rate from satellites to ground by orders of magnitude,” said Sascha.

“The next generation of big data-gathering satellites would be able to get critical information to the ground faster.”

With humans also expected to return to the lunar surface within the decade, this type of technology would provide as close to real-time communication, telemetry, and risk data for effective, and potentially, life-saving informed decision making.

Another is to utilise the technology for space-based satellite-to-satellite communications, or even out to the interplanetary robotic probes exploring the Solar system, who gather vast quantities of data that requires transmission over even grander distances.

“There is the potential to use the technology we have developed for ground to space transmissions. There are technical challenges, such as pointing difficulties and enormous power losses, that we are currently working on with the hope of achieving a ground to space transmission,” said Ben.

Working as part of an international team during the COVID pandemic brings its own challenges, but the researchers from Australia were celebrating their wins as results started emerging.

Alongside Ben and Sascha, Dr. David Gozzard (Forrest Fellow at ICRAR/UWA) commented on how exciting the opportunity was to work with international agencies.

“I am super excited to be working with CNES and SYRTE. It was fantastic to get to go to Toulouse, one of the world’s biggest centres for space and aviation research and development, to do this work,” he said.

“We’re very keen to continue working with our friends at CNES and SYRTE, continuing to modify and improve our technology – so that over the next few years we can test them on high altitude balloons, drones, and eventually, on a satellite.”

Echoing David’s thoughts, Ben described also working on the ground components of the technology to build the end-to-end capability of this new, laser technology.

“We have been working on improving the active optical terminals and have recently been working towards adapting the system for use over future ground-to-satellite links. At the UWA campus in Perth, we are involved with the development of an optical ground station with the potential to enable vertical transmissions to satellites. This could enable novel physics experiments, as well as aid in high-speed optical communications,” said Ben.

With the science mega-project, the Square Kilometre Array – which will be partially constructed in Western Australia now starting to take shape, its these early pioneering increments and improvements of technology, that will allow scientists 10 years from now to be the beneficiaries of future discoveries through the underlying capabilities that are on display today.

We’d say Einstein would be pretty chuffed with how things are coming along.